From NPR (emph. added):

NPR and South Carolina ETV Radio have cancelled their national Republican presidential debate, scheduled for Wednesday, January 16 in Columbia, S.C.

The audio-only debate, to be broadcast on NPR Member stations nationally and webcast, was set to air live from 2:00-4:00PM (ET).

"Ultimately, not enough candidates were willing to participate," said Andi Sporkin, Vice President for Communications, NPR. "We look forward to working with them through other NPR News programming over the coming months and during the general election."